Re: [PATCH v2 0/6] perf: Introduce extended syscall error reporting

From: Johannes Berg
Date: Tue Aug 25 2015 - 05:34:22 EST

On Tue, 2015-08-25 at 11:17 +0200, Ingo Molnar wrote:
> If we do that then we don't even have to introduce per system call error code
> conversion, but could unconditionally save the last extended error info in the
> task struct and continue - this could be done very cheaply with the linker trick
> driven integer ID.
> I.e. system calls could opt in to do:
> > return err_str(-EBUSY, "perf/x86: BTS conflicts with active events");
> and the overhead of this would be minimal, we'd essentially do something like this
> to save the error:
> > current->err_code = code;
> where 'code' is a build time constant in essence.
> We could use this even in system calls where the error path is performance
> critical, as all the string recovery and copying overhead would be triggered by
> applications that opt in via the new system call:
> > struct err_desc {
> > const char *message;
> > const char *owner;
> > const int code;
> > };
> > SyS_err_get_desc(struct err_desc *err_desc __user);
> [ Which could perhaps be a prctl() extension as well (PR_GET_ERR_DESC): finally
> some truly matching functionality for prctl(). ]
> Hm?

That's neat in a way, but doesn't work in general I think.

Considering the wifi case, or more generally any netlink based
protocol, the syscall (sendmsg) won't return an error, but a subsequent
recvmsg() (which also won't return an error) returns an error message
[in the sense of a protocol message, not a human readable message] to a
buffer provided by the application.
However, this message can be extended relatively easily to include the
string information, but the syscall/prctl wouldn't work since the
syscalls didn't actually fail.

However, it could possibly help with the namespace/module issue if you
also store THIS_MODULE (or perhaps instead a pointer to the module's
error table) in the task. Again not in the netlink case though, I
think, that will always require special handling [although there it
could be stored away in the socket or so, similar to the task]

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